<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box {
            width: 0;
            height: 0;
            border-top: 10px solid red;
            /*border-bottom: 10px solid blue;*/
            border-left: 10px solid transparent;
            border-right: 10px solid transparent;
        }
    </style>
</head>
<body>
<div class="box"></div>
</body>
</html>